CSS n00b: em vs. pt?

Ok, so my WP blogs look differently in FF and IE. I use FF and they look perfect – font sizes are small. In IE though, when set to ‘medium’ font size (the default for most people), text size is too large and the site looks, to me, goofy:

So, being the CSS hax0r that I am, I looked at the code and realize I don’t know my em from my pt. Whiskey Tango Foxtrot! It seems that IE is having problems with em. Can’t I just change the CSS to use pt with decimal equivalents (9.5pt etc.) or is that a bad idea?